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Astronomy 182: Origin


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Title: Astronomy 182: Origin

Astronomy 182 Origin Evolution of the Universe
Lecture 8

For next week Essay on Greene, chapter 3 due
April 23. Reading for this material Ferris,
chapter 5

The Dark Side of the Universe, Part I
Dark Matter
Dark Matter and the Density of the Universe
To determine the cosmic mass density ?0 (and thus
the fate of the Universe), we cannot just
inventory the luminous objects we see, because
evidence indicates Most of the
Mass in the Universe is DARK Dark Matter any
matter whose existence is inferred solely
from its gravitational effects (i.e., does not
emit light)
The 2 Dark Matter Problems
Observations indicate ?visible matter 0.01
?baryons 0.04 ?dark matter 0.3
Dark Baryonic matter composed of protons,
neutrons, (more fundamentally of quarks)
Dominant component of Dark Matter is
Non-baryonic requires a new component beyond
Finding Dark Matter by Gravity
Consider a test particle of mass m orbiting a
body of mass M
Kinetic Energy mv2/2

Potential Energy ?GMm/d (Newton) Conservation
of Energy Kinetic Potential Etotal
mv2/2 ? GMm/d E For gravitationally bound
systems, Etotal is comparable to the terms on
the LHS, thus
v2 GM/d Virial
Theorem Use velocities of test particles to
probe the mass
Aside use this to derive Schwarzschild radius
Tutorial Dark Matter (evidence)
Website http//
torial/dm0.html (created by Jonathan Dursi,
current Astronomy graduate student at U.
Chicago) See Cluster simulation to show
relation of Velocities with Mass http//www.astr
Dark Matter in Galaxy Clusters
  • Historically, first evidence for Dark Matter
  • Use galaxies as test particles measure
  • their velocities and infer the cluster mass.
    Found galaxies
  • moving significantly faster than expected (v
    1000 km/sec)
  • if all the mass were in the luminous galaxies
    in the cluster.
  • Evidence accumulated in 1930s by Fritz Zwicky,
  • studied galaxy motions (via redshifts) in
    the Coma cluster
  • Dark matter in clusters contains 10 times more
    mass than the
  • visible galaxies in them. Cluster galaxies
    are like raisins in a
  • raisin cake of dark matter.

Coma cluster about 70 Mpc distant
Fritz Zwicky Caltech astronomer irascible
character (aka pain in the ) came up with both
groundbreaking and crazy ideas, the latter of
which impeded acceptance of the former
Dark Matter from Spiral Galaxy Rotation Curves
  • Strongest evidence for Dark Matter
  • Use stars and gas in a galaxy as test
    particles measure
  • their velocities and infer the galaxy mass
  • Evidence accumulated starting in 1970s by Vera
  • and collaborators
  • Luminous galaxies embedded in Dark (matter) halos
  • extend well beyond the visible disk and
    contain from
  • several to 10 times more mass than the
    visible parts
  • (stars gas).

Edge-on Spiral (disk) Galaxy
Measure rotation curve (circular speed of stars
gas in the disk as function of distance from the
center) using redshift (subtract out galaxys
uniform Hubble recession velocity)
rotation velocity
Dark halo
Observed flat, M d
Keplerian v d-1/2
Typical rotation speed 200 km/sec and visible
disk size 10 kpc Halos extend 10 times farther
See Dursi demo on rotation curves http//www.ast
Evidence for Dark Matter from Gravitational
Dynamical evidence for Dark Matter DM
affects the motions of gas and stars (in
galaxies) and galaxies themselves (in
clusters) Lensing evidence for Dark Matter
DM curves spacetime and thus bends light rays
coming from background sources
Clusters of Galaxies Size 1025 cm Megaparsec
(Mpc) Mass 1015 Msun Largest
gravitationally bound objects galaxies, gas,
dark matter
Cluster of Galaxies
giant arcs are galaxies behind the cluster,
gravitationally lensed by it
Helen Frieman b. 9/20/99
Helen behind a massive Body (in this case, a
Black Hole) Amount of shear of background
image(s) is proportional to the foreground
Lensing Movie http//
Gravitational Lensing Geometry
Strong Lensing multiple images or pronounced
shear of images (lens is close to line of
sight to the source) Weak Lensing small shear of
images, must be inferred from studying large
number of source images behind the lens
(No Transcript)
Lensing as an effect of curved space(time) on
Gravitational Lensing Probes the Mass
Distribution in Galaxy Clusters ? Determine the
amount of Dark Matter in these systems
HST image
Mapping the Mass in a Cluster of Galaxies via
Weak Gravitational Lensing Most of the Mass in
these Systems is Dark Matter
Multiple Evidence for Dark Matter in Clusters
1. Dynamics (motions) of galaxies in clusters
(Zwicky) 2. Giant arcs (strong lensing) and weak
lensing 3. Measure intensity and temperature of
hot, X-ray emitting gas in the cluster (but
mostly outside the cluster galaxies) X-ray
Temperature analogous to velocities of galaxies T
v2 4. Sunyaev-Zeldovich effect measure
deviation in CMB temperature through the
cluster due to scattering of CMB photons to
higher energies by the hot intracluster
gas. These methods are in broad agreement on
cluster masses and indicate that ?matter
0.2 - 0.3
Combined X-ray and Sunyaev- Zeldovich Effect Ma
p of a Galaxy Cluster
Evidence for Dark Matter from Statistical Weak
Lensing by Galaxies
Galaxy-galaxy lensing as a probe of Galaxy DM
Halos Measure the shapes of a large sample of
distant background galaxies, the images of
which are sheared by the Dark Matter halos of
a foreground population of galaxies instead
of probing the mass of individual objects, this
method probes the average mass of a
population of galaxies
Lensing of intrinsically spherical galaxies
ellipticities exaggerated
Foreground galaxy
Background Source shape
Lensing of real (elliptically shaped) galaxies
Foreground galaxy
Background Source shape
Repeat for a large number of foreground galaxies
Galaxy-Galaxy Lensing in early SDSS
Data Galaxy-mass Correlation function 31,000
foreground galaxies with measured redshifts
106 background galaxy shapes
(18ltrlt22) Fischer, etal McKay, etal
from foreground galaxy
December 14, 1999
Basic Dark Matter Questions
How much is there? What is the value of ??
Current evidence suggests 0.3. Where is it?
Is it just clustered with the luminous material?
Not precisely, since Dark halos extend beyond
luminous galaxies. Are there completely dark
galaxies or clusters? (Some weak lensing
studies suggest existence of dark
clumps.) What is it? Evidence from Big Bang
Nucleosynthesis suggests most of the DM is
not made of baryons (i.e., protons, neutrons,
etc). Theory suggests it is an as yet
undiscovered, Weakly Interacting Elementary
particle (WIMP). Ultimate Copernican principle
Were not even made of the central stuff of the
Baryonic Dark Matter
Big Bang Nucleosynthesis indicates ?baryons
0.04, substantially larger than the
density of luminous matter (stars gas in
galaxies clusters). Some possibilities for
hiding baryons in dark objects MASSIVE
(MACHOs) Brown Dwarfs
objects with mass lt 1/10 that of the Sun,
do not get hot enough to undergo thermonuclear
burning, so they do not shine (e.g., the
planet Jupiter) White Dwarfs old stars of
relatively low mass that have used up
their nuclear fuel and collapsed to degenerate
objects Neutron Stars intermediate mass stars
that have used up their fuel Black Holes Likely
end-state of Higher-mass stars
Searching for MACHOs in the Milky Way Halo via
microlensing of stars in a nearby galaxy (Large
Magellanic Cloud) monitor 106 stars over time
Duration of brightening constrains MACHO mass
Background star appears brighter when lensed by
the foreground MACHO (note angular separation
of the 2 images too small to observe)
MACHOs in our Halo
Several experiments carried out over the last
decade, monitoring the brightness of millions
of stars in nearby galaxies. A number of
microlensing events have now been detected,
though the interpretation in terms of Dark
Matter is still controversial. MACHOs could
compose 30 of the Milky Ways Dark Halo
mass (or they could be a separate disk
population) Inferred MACHO mass is 1/2 the
mass of the Sun, suggesting they may be White
Dwarfs. Not clear how these objects concord
with our current understanding of star formation.
Non-baryonic Dark Matter
Evidence on the amount of Dark Matter and
constraints from Big Bang Nucleosynthesis
indicate the Universe is dominated by an
Exotic Dark Matter component, made of some new,
as-yet undiscovered elementary particle. In
order for it to be dark matter, this particle
should be weakly interacting (i.e., not
interact with or emit light or other
electromagnetic radiation) and naturally have an
abundance in the Universe which yields ?dark
matter 0.3. Models of Elementary
Particle Physics provide a number of Dark
Matter candidates, new particles with these
requisite properties WEAKLY INTERACTING
(Some) Dark Matter Candidates
Neutrinos (mass few eV 10-5 electrons
mass) known to exist but not (yet) known to
have (the requisite) mass (could be some of
the DM, but not all of it) Supersymmetric
Particles (LSP) (mass 10-100 proton mass)
Favorite candidate of particle physics
theorists Axions (mass 10-5 eV 10-10
electron mass) Hypothetical particle that
arises in theories that seek to explain
certain features of the strong interactions Note
all these candidates (and others) involve (so
far unproved) physics beyond the well-tested
Standard Model of Particle Physics. Thus,
determining the nature of the Dark Matter should
tell us about fundamental physics.
Detecting Dark Matter Neutrinos
Substantial experimental efforts underway to
determine the masses of the neutrinos (there
are 3 types of neutrinos we know of, ?e ?? ??
) use neutrinos generated (i) by the Sun,
(ii) by cosmic rays hitting the Earths
atmosphere, and (iii) by particle
accelerators (like Fermilab) on Earth. Massive
neutrinos transmute (oscillate) into each other
as they travel, depending on their masses.
Requires massive underground detectors since
neutrinos interact only weakly and to avoid
backgrounds caused by other particles. Note
about 1014 neutrinos from the Sun pass through
your body every second
Detecting Dark Matter SUSY WIMPs
Weakly interacting (e.g., Supersymmetric) Dark
Matter particles travelling through the Milky
Way halo as the Earth orbits through the
galaxy, it continually runs into this spray of
particles. Occasionally, one of these WIMPs
knocks into a nucleus in a detector, and one
can look for the signature of such an event. For
example, in a bolometric detector (operated
at very low Temperature), the recoiling
nucleus scatters with other nuclei in the
detector, and the whole detector heats up by a
miniscule but measurable amount. Efforts now
underway to detect these particles. Also search
for Supersymmetric particles in particle
accelerators such as CERN (in Geneva) and
Fermilab (in Batavia). About 109 halo WIMPs
would pass through your body every second.